diff --git a/src/client.js b/src/client.js
index 162f596d8..c744ff33d 100644
--- a/src/client.js
+++ b/src/client.js
@@ -550,6 +550,9 @@ function _decryptEvent(client, event) {
     try {
         client._crypto.decryptEvent(event);
     } catch (e) {
+        console.warn(
+            `Error decrypting event (id=${event.getId()}): ${e}`,
+        );
         if (!(e instanceof Crypto.DecryptionError)) {
             throw e;
         }
diff --git a/src/crypto/DeviceList.js b/src/crypto/DeviceList.js
index ebc0a59de..49b4a4179 100644
--- a/src/crypto/DeviceList.js
+++ b/src/crypto/DeviceList.js
@@ -404,7 +404,7 @@ function _updateStoredDeviceKeysForUser(_olmDevice, userId, userStore,
     const signKeyId = "ed25519:" + deviceId;
     const signKey = deviceResult.keys[signKeyId];
     if (!signKey) {
-        console.log("Device " + userId + ":" + deviceId +
+        console.warn("Device " + userId + ":" + deviceId +
             " has no ed25519 key");
         return false;
     }
@@ -414,8 +414,8 @@ function _updateStoredDeviceKeysForUser(_olmDevice, userId, userStore,
     try {
         olmlib.verifySignature(_olmDevice, deviceResult, userId, deviceId, signKey);
     } catch (e) {
-        console.log("Unable to verify signature on device " +
-            userId + ":" + deviceId + ":", e);
+        console.warn("Unable to verify signature on device " +
+            userId + ":" + deviceId + ":" + e);
         return false;
     }
 
@@ -431,7 +431,7 @@ function _updateStoredDeviceKeysForUser(_olmDevice, userId, userStore,
             // best off sticking with the original keys.
             //
             // Should we warn the user about it somehow?
-            console.warn("Ed25519 key for device" + userId + ": " +
+            console.warn("Ed25519 key for device " + userId + ":" +
                deviceId + " has changed");
             return false;
         }
diff --git a/src/crypto/algorithms/base.js b/src/crypto/algorithms/base.js
index 407e09d96..bcf536fa2 100644
--- a/src/crypto/algorithms/base.js
+++ b/src/crypto/algorithms/base.js
@@ -148,14 +148,42 @@ DecryptionAlgorithm.prototype.importRoomKey = function(session) {
 /**
  * Exception thrown when decryption fails
  *
+ * @alias module:crypto/algorithms/base.DecryptionError
  * @constructor
- * @param {string} msg message describing the problem
+ * @param {string} msg user-visible message describing the problem
+ *
+ * @param {Object=} details key/value pairs reported in the logs but not shown
+ *   to the user.
+ *
  * @extends Error
  */
-module.exports.DecryptionError = function(msg) {
+const DecryptionError = function(msg, details) {
+    this.name = 'DecryptionError';
     this.message = msg;
+    this.details = details;
 };
-utils.inherits(module.exports.DecryptionError, Error);
+utils.inherits(DecryptionError, Error);
+
+/** override the string used when logging
+ *
+ * @returns {String}
+ */
+DecryptionError.prototype.toString = function() {
+    let result = this.name + '[msg: '+ this.message;
+
+    if (this.details) {
+        result += ', ' +
+            Object.keys(this.details).map(
+                (k) => k + ': ' + this.details[k],
+            ).join(', ');
+    }
+
+    result += ']';
+
+    return result;
+};
+
+module.exports.DecryptionError = DecryptionError;
 
 /**
  * Exception thrown specifically when we want to warn the user to consider
diff --git a/src/crypto/algorithms/megolm.js b/src/crypto/algorithms/megolm.js
index df4fdb5af..f3a76eb1b 100644
--- a/src/crypto/algorithms/megolm.js
+++ b/src/crypto/algorithms/megolm.js
@@ -544,7 +544,11 @@ MegolmDecryption.prototype.decryptEvent = function(event) {
         if (e.message === 'OLM.UNKNOWN_MESSAGE_INDEX') {
             this._addEventToPendingList(event);
         }
-        throw new base.DecryptionError(e);
+        throw new base.DecryptionError(
+            e.toString(), {
+                session: content.sender_key + '|' + content.session_id,
+            },
+        );
     }
 
     if (res === null) {
@@ -552,6 +556,9 @@ MegolmDecryption.prototype.decryptEvent = function(event) {
         this._addEventToPendingList(event);
         throw new base.DecryptionError(
             "The sender's device has not sent us the keys for this message.",
+            {
+                session: content.sender_key + '|' + content.session_id,
+            },
         );
     }
 
@@ -593,26 +600,27 @@ MegolmDecryption.prototype._addEventToPendingList = function(event) {
  * @param {module:models/event.MatrixEvent} event key event
  */
 MegolmDecryption.prototype.onRoomKeyEvent = function(event) {
-    console.log("Adding key from ", event);
     const content = event.getContent();
     const senderKey = event.getSenderKey();
+    const sessionId = content.session_id;
 
     if (!content.room_id ||
-        !content.session_id ||
+        !sessionId ||
         !content.session_key ||
         !senderKey
        ) {
-        console.error("key event is missing fields");
+        console.error(`key event is missing fields`);
         return;
     }
 
+    console.log(`Adding key for megolm session ${senderKey}|${sessionId}`);
     this._olmDevice.addInboundGroupSession(
-        content.room_id, senderKey, content.session_id,
+        content.room_id, senderKey, sessionId,
         content.session_key, event.getKeysClaimed(),
     );
 
     // have another go at decrypting events sent with this session.
-    this._retryDecryption(senderKey, content.session_id);
+    this._retryDecryption(senderKey, sessionId);
 };
 
 
diff --git a/src/crypto/algorithms/olm.js b/src/crypto/algorithms/olm.js
index 99fb64c29..f8715b1e2 100644
--- a/src/crypto/algorithms/olm.js
+++ b/src/crypto/algorithms/olm.js
@@ -174,12 +174,12 @@ OlmDecryption.prototype.decryptEvent = function(event) {
     try {
         payloadString = this._decryptMessage(deviceKey, message);
     } catch (e) {
-        console.warn(
-            "Failed to decrypt Olm event (id=" +
-                event.getId() + ") from " + deviceKey +
-                ": " + e.message,
+        throw new base.DecryptionError(
+            "Bad Encrypted Message", {
+                sender: deviceKey,
+                err: e,
+            },
         );
-        throw new base.DecryptionError("Bad Encrypted Message");
     }
 
     const payload = JSON.parse(payloadString);
@@ -187,22 +187,18 @@ OlmDecryption.prototype.decryptEvent = function(event) {
     // check that we were the intended recipient, to avoid unknown-key attack
     // https://github.com/vector-im/vector-web/issues/2483
     if (payload.recipient != this._userId) {
-        console.warn(
-            "Event " + event.getId() + ": Intended recipient " +
-            payload.recipient + " does not match our id " + this._userId,
-        );
         throw new base.DecryptionError(
             "Message was intented for " + payload.recipient,
         );
     }
 
-    if (payload.recipient_keys.ed25519 !=
-               this._olmDevice.deviceEd25519Key) {
-        console.warn(
-            "Event " + event.getId() + ": Intended recipient ed25519 key " +
-            payload.recipient_keys.ed25519 + " did not match ours",
+    if (payload.recipient_keys.ed25519 != this._olmDevice.deviceEd25519Key) {
+        throw new base.DecryptionError(
+            "Message not intended for this device", {
+                intended: payload.recipient_keys.ed25519,
+                our_key: this._olmDevice.deviceEd25519Key,
+            },
         );
-        throw new base.DecryptionError("Message not intended for this device");
     }
 
     // check that the original sender matches what the homeserver told us, to
@@ -210,23 +206,19 @@ OlmDecryption.prototype.decryptEvent = function(event) {
     // (this check is also provided via the sender's embedded ed25519 key,
     // which is checked elsewhere).
     if (payload.sender != event.getSender()) {
-        console.warn(
-            "Event " + event.getId() + ": original sender " + payload.sender +
-            " does not match reported sender " + event.getSender(),
-        );
         throw new base.DecryptionError(
-            "Message forwarded from " + payload.sender,
+            "Message forwarded from " + payload.sender, {
+                reported_sender: event.getSender(),
+            },
         );
     }
 
     // Olm events intended for a room have a room_id.
     if (payload.room_id !== event.getRoomId()) {
-        console.warn(
-            "Event " + event.getId() + ": original room " + payload.room_id +
-            " does not match reported room " + event.room_id,
-        );
         throw new base.DecryptionError(
-            "Message intended for room " + payload.room_id,
+            "Message intended for room " + payload.room_id, {
+                reported_room: event.room_id,
+            },
         );
     }
 
diff --git a/src/sync.js b/src/sync.js
index facf5d6ed..237a1e24a 100644
--- a/src/sync.js
+++ b/src/sync.js
@@ -360,9 +360,6 @@ SyncApi.prototype.getSyncState = function() {
  * Main entry point
  */
 SyncApi.prototype.sync = function() {
-    debuglog("SyncApi.sync: starting with sync token " +
-             this.client.store.getSyncToken());
-
     const client = this.client;
     const self = this;
 
@@ -511,11 +508,14 @@ SyncApi.prototype._sync = function(syncOptions) {
     // normal timeout= plus buffer time
     const clientSideTimeoutMs = this.opts.pollTimeout + BUFFER_PERIOD_MS;
 
+    debuglog('Starting sync since=' + syncToken);
     this._currentSyncRequest = client._http.authedRequest(
         undefined, "GET", "/sync", qps, undefined, clientSideTimeoutMs,
     );
 
     this._currentSyncRequest.done(function(data) {
+        debuglog('Completed sync, next_batch=' + data.next_batch);
+
         // set the sync token NOW *before* processing the events. We do this so
         // if something barfs on an event we can skip it rather than constantly
         // polling with the same token.
@@ -666,8 +666,10 @@ SyncApi.prototype._processSyncResponse = function(syncToken, data) {
                         toDeviceEvent.getType() == "m.room.message" &&
                             content.msgtype == "m.bad.encrypted"
                     ) {
-                        console.warn(
-                            "Unable to decrypt to-device event: " + content.body,
+                        // the mapper already logged a warning.
+                        console.log(
+                            'Ignoring undecryptable to-device event from ' +
+                                toDeviceEvent.getSender(),
                         );
                         return;
                     }